Sets of documents typically exist in an order. For example, a pre-existing collection of documents (e.g., a legal case file, a police file, etc.) is typically organized in chronological or reverse-chronological order. As another example, search results are normally presented in descending order of relevance to a query.
People typically read a set of documents in the order in which they are presented. However, it may be inefficient to read the documents in this order. Documents that appear near each other in the given order may contain redundant information, and reading several documents that have the same, or similar, information provides the reader with relatively little new information for the amount of time invested.